1. Field of the Invention
This invention relates to electrophotography, especially a drying method and apparatus for use with liquid toners.
2. Background of the Art
Electrophotography forms the technical basis for various well known imaging processes, including photocopying and some forms of laser printing. The basic electrophotographic process involves placing a uniform electrostatic charge on a photoconductor element, imagewise exposing the photoconductor element to activating electromagnetic radiation (also referred to herein as “light”) and thereby dissipating the charge in the exposed areas to form an electrostatic latent image, developing the resulting electrostatic latent image with a toner, and transferring the toner image from the photoconductor element to a final substrate, such as paper, either by direct transfer or via an intermediate transfer material. The direct or intermediate transfer typically occurs by one of two methods: electrostatic assist (electrostatic transfer) or elastomeric assist (adhesive transfer). The effectiveness of adhesive transfer is controlled by several variables including surface energy, temperature, and pressure. Electrostatic transfer is also affected by surface energy, temperature, and pressure, but the primary driving force causing the toner image to be transferred to the final substrate is via electrostatic forces.
The structure of a photoconductor element generally may be a continuous belt, which is supported and circulated by rollers, or a rotatable drum. All photoconductor elements have a photoconductive layer which transports charge (either by an electron transfer of charge transfer mechanism) when the photoconductive layer is exposed to activating electromagnetic radiation or light. The photoconductive layer is generally affixed to an electroconductive support. The surface of the photoconductor element is either negatively or positively charged such that when activating electromagnetic radiation strikes a region of the photoconductive layer, charge is conducted through the photoconductor element in that region to neutralize or reduce the surface potential in the illuminated region. An optional barrier layer may be used over the photoconductive layer to protect the photoconductive layer and extend the service life of the photoconductive layer. Other layers, such as adhesive layers or priming layers or charge injection blocking layers are also used in some photoconductor elements. A release layer may be used to facilitate transfer of the image from the photoconductor element (also referred to herein as the photoreceptor) to either the final substrate, such as paper, or to an intermediate transfer element.
Typically, a positively charged toner is attracted to those areas of the photoconductor element which retain a negative charge after the imagewise exposure, thereby forming a toner image which corresponds to the electrostatic latent image. The toner need not be positively charged, although that charge form or a neutral charge is preferable. Some toners (irrespective of their charge) may be attracted to the areas of the photoconductor element where the charge has been dissipated. The toner may be either a powdered material comprising a blend or association of polymer and colored particulates, typically carbon, or a liquid material of finely divided solids dispersed in an insulating liquid frequently referred to as a carrier liquid.
Generally, the carrier liquid is a hydrocarbon that has a low dielectric constant (e.g., less than 3) and a vapor pressure sufficiently high to ensure rapid evaporation of solvent following deposition of the toner onto a photoreceptor, transfer belt, and/or receptor sheet. Rapid evaporation is particularly important for cases in which multiple colors are sequentially deposited and/or transferred to form a single image. Examples of such carrier liquids include NORPAR™ and ISOPAR™ solvents from Exxon Chemical Company.
Liquid toners are often preferable because they are capable of giving higher resolution images and require lower energy for image fixing than do dry toners. However, excess carrier liquid which is transferred to the photoconductor element can create a variety of problems. When either the elastomeric or adhesive transfer mechanism is being used, removal of excess carrier liquid is especially important. The excess carrier liquid can blot or stain the image or can cause smudging or streaking of the images. In addition, if excess carrier liquid is not removed, additional energy will be required at the image fixing step to volatilize the excess carrier liquid. Also, removal of the excess carrier liquid generally leads to improved image clarity and image density.
A variety of methods have been employed to remove excess carrier liquid from a developed toner image. These methods include squeegee rolls, air knives, corona discharge, vacuum removal, and absorption.
U.S. Pat. No. 5,420,675 to Thompson et al. discloses the use of a film forming roll which has a thin, outer layer which is compatible (referred to as ‘philic’) with the carrier liquid and an inner layer which is carrier liquid-phobic and compressible. The film forming roll of that patent is maintained in contact with a single heating roll. The carrier liquid entrained in the film forming roll is removed by heating the liquid to a temperature greater than or equal to the flashpoint of the liquid.
U.S. Pat. No. 5,552,869 to Schilli et al. discloses a drying method and apparatus for electrophotography using liquid inks. The drying apparatus removes excess carrier liquid from an image produced by liquid electrophotography on a moving organophotoreceptor. The system includes a drying roll that contacts the organophotoreceptor, with an outer layer that absorbs and desorbs the carrier liquid and an inner layer having a Shore A hardness of 10 to 60 which is carrier liquid-phobic, and a heating means to increase the temperature of the drying roll to no more than 5° C. below the flashpoint of the carrier liquid. In one embodiment, the heating means includes two hot rolls and the system further includes a cooling means that cools the drying roll.
U.S. Pat. No. 5,736,286 to Kaneko et al. discloses the employment of a drying belt to remove carrier fluids in liquid inks.